A sliding door device for a vehicle may include a sliding door opening/closing a door opening formed in a vehicle body, a door rail mounted on the sliding door, a vehicle body rail mounted on the vehicle body, a central support structure for movably supporting the sliding door, a hook releasably locking the central support structure to the door rail, and a lower support structure for movably supporting a lower portion of the sliding door. The central support structure includes a central slider. The lower support structure includes a lower slider, a gear device, a link mechanism, a chain, and a chain tensioner.